1. Field of the Invention
This invention relates to a photometric apparatus applicable to photographing with use of an electronic camera for recording images of an object in a recording medium such as a memory card by photo-electrically converting a light image of the object into electronic data, such as digital still camera, and digital video recorder.
2. Description of the Related Art
A camera records a light image of an object by focusing the light image on a focal plane by a lens and by exposing the focused image on a photosensitive member such as a film disposed on the focal plane. Since a film has photosensitive characteristics such as photosensitivity and characteristic curve, it is required to provide an optimal exposure amount for the film according to the characteristic of the film in order to obtain an optimal record image.
Generally, the range of brightness of light in the nature is remarkably wide compared to a reproducible range of brightness on a film. Accordingly, it is impossible to record the whole range of brightness of natural light on a film. In view of this, it has been a common practice in photographing to determine an exposure amount based on an idea that an object to be recorded has a reflectance of 18%, which is a mean value of light distribution in the nature, and that such an object is recorded at a density in an intermediate portion of the characteristic curve of the film and to designate the determined exposure amount as an optimal exposure amount.
As is well known, parameters for controlling the exposure amount of a camera include photosensitivity of a film, exposure period and aperture of the camera, and brightness of an object. Even if the brightness of an object is known, it is required to determine an exposure period or shutter speed for obtaining an optimal exposure and an aperture depending on the photosensitivity of the film. Hereafter, the shutter speed and the aperture are referred to as “exposure control values”. It is not easy to calculate these exposure control values. In view of this, there has been produced a photometer for measuring a brightness of an object to calculate an optimal exposure control value based on the measurement result and the photosensitivity of the film.
Generally, photometers for measuring incident light on an object, and photometers for measuring reflected light from an object are used. These photometers are designed to calculate exposure control values (shutter speed Tv, aperture value Av [APEX value]) based on the following respective Equations (1), (2) for display.            ⁢                    Ev                    =                              Bv          +          Sv                                                                       ⁢                                    (              1              )                        ⁢                                                   ⁢                          (                              Equation                ⁢                                                                   ⁢                using                ⁢                                                                   ⁢                reflected                ⁢                                                                   ⁢                light                            )                                                                                                 =                              Iv          +          Sv                                                                       ⁢                                    (              2              )                        ⁢                                                   ⁢                          (                              Equation                ⁢                                                                   ⁢                using                ⁢                                                                   ⁢                incident                ⁢                                                                   ⁢                light                            )                                                                                                 =                              Av          +          Tv                                                                       ⁢                      (            3            )                    ⁢                                                       ⁢         where                Ev: exposure value        Sv: ISO sensitivity of film        Bv: brightness of object        Iv: illuminance of object        
The conventional photometer is adapted in photographing with a camera using a silver halide film. In photographing with use of such a photometer, an exposure value is calculated as an optimal exposure amount for a film. Specifically, in a silver halide film, after a latent image is formed by exposure, developing and fixing are performed prior to reproduction of a photographed image on the film. In such a processing, it is impossible to determine a correspondence between a brightness distribution of an object light image and a density distribution of a photographed image on the film prior to image reproduction. In view of this, an exposure amount of the film is determined based on the idea that an object image having a reflectance of 18% is recorded with a density in an intermediate portion of the characteristic curve of the film to facilitate setting of exposure control values.
In recent years, there have been rapidly developed electronic cameras incorporated with a solid-state image pickup device like CCD, such as digital still cameras and digital video recorders. Some of the electronic cameras have accomplished an image resolution as high as a corresponding performance of a camera using a silver halide film. The electronic cameras record an object image in a semiconductor memory or its equivalent by converting a brightness distribution of an object light image to digital data of gradations ranging from 0 to 255 in the case where 8-bit is used. In other words, the electronic cameras record an object light image in terms of gradation values corresponding to a density distribution of a photographed image on a film.
The above reveals that the electronic cameras can show a correspondence between a brightness distribution of an object light image (distribution in input data) and a density distribution of an image recorded in a recording medium (distribution in output data) and enable a photographer to evaluate an exposure in terms of a record image in photographing. Obtaining such information is desirable for the photographer because the photographer can evaluate the exposure in terms of a record image (such as a gradation characteristic and a gradation range of a record image) as well as in terms of exposure control values.
However, the conventional photometer has a limitation that merely an exposure control value is calculated based on an optimal exposure amount on a film. Applying the conventional photometer to an electronic camera such as a digital still camera fails to obtain information that enables a photographer to evaluate an exposure in terms of a record image. Particularly, a solid-state image pickup device of an electronic camera has a narrower dynamic range than a silver halide film, and a characteristic of the solid-state image pickup device corresponding to a characteristic curve of a silver halide film is different from that of the silver halide film. Accordingly, it is not easy to apply measurement results of the conventional photometer to an electronic camera while utilizing the techniques and knowledge concerning exposure control with respect to a silver halide film. Despite the fact that there has been a demand for a photometric apparatus for use with an electronic camera considering the recording system of the electronic camera, such a photometric apparatus has not been proposed or available.